A new incubation system for the measurement of copepod egg production and egg hatching success in the field

Abstract

The design and testing of an incubation system for studying copepod egg production and hatching success that allows undisturbed incubation and monitoring of eggs under field conditions for extended periods of time is described. The incubation system was tested with copepods from polar (Weddell Sea), temperate (Irish Sea) and subtropical (Atlantic Coast, Gulf of Mexico and Gulf Stream) seas between 1993 and 1995. Individual female copepods sorted from plankton tows were incubated at ambient temperature and photoperiod (5% incident light) for 24 h in prescreened (size determined by egg diameter) water from the sampling location. The incubation system consists of an upper spawning chamber that houses the female and a lower brooding chamber where the eggs incubate. After 24 h, the chambers are separated and the eggs are permitted to incubate some 24–96 h longer while being monitored under a microscope to determine the onset of hatching. When nauplii are detected in all containers, eggs and nauplii are incubated an additional 24 h to allow adequate time for the eggs released near the end of the spawning period to hatch. Nine experiments were conducted to compare our incubator and protocol with a “standard” technique for measuring egg production. Mean egg production rates measured by both techniques were not statistically distinguishable in 8 of the 9 runs. Hatching success was variable between and within species. Mean hatching success was surprisingly low (62.7%) when taken over the eight species, which suggests that a substantial portion of egg production may not be viable. Apparently, some egg disintegrated during the extended incubation in 43% of our experiments, emphasizing that when measuring hatching success, it is important to perform counts upon the removal of the female so as not to underestimate the rate of egg production or to overestimate hatching success. The present technique permits one to estimate egg production, as well as, hatching success and the disintegration of delicate non-viable eggs.